Description: The first Cenozoic ice sheets initiated in Antarctica from the Gamburtsev Subglacial Mountains and other highlands as a result of rapid global cooling ~34 million years ago. In the subsequent 20 million years, at a time of declining atmospheric carbon dioxide concentrations and an evolving Antarctic circumpolar current, sedimentary sequence interpretation and numerical modelling suggest that cyclical periods of ice-sheet expansion to the continental margin, followed by retreat to the subglacial highlands, occurred up to thirty times. These fluctuations were paced by orbital changes and were a major influence on global sea levels. Ice-sheet models show that the nature of such oscillations is critically dependent on the pattern and extent of Antarctic topographic lowlands. Here we show that the basal topography of the Aurora Subglacial Basin of East Antarctica, at present overlain by 2-4.5 km of ice, is characterized by a series of well-defined topographic channels within a mountain block landscape. The identification of this fjord landscape, based on new data from ice-penetrating radar, provides an improved under¬standing of the topography of the Aurora Subglacial Basin and its surroundings, and reveals a complex surface sculpted by a succession of ice-sheet configurations substantially different from today's. At different stages during its fluctuations, the edge of the East Antarctic Ice Sheet lay pinned along the margins of the Aurora Subglacial Basin, the upland boundaries of which are currently above sea level and the deepest parts of which are more than 1 km below sea level. Although the timing of the channel incision remains uncertain, our results suggest that the fjord landscape was carved by at least two ice- flow regimes of different scales and directions, each of which would have over-deepened existing topographic depressions, reversing valley floor slopes.

Description: Gravity anomalies provide a tool to study crustal structure, effective elastic thickness, and isostatic and tectonic processes. Over the last 10 years major airborne gravity surveys were flown by the international community over several Antarctic frontiers. The longer-wavelength Antarctic gravity anomaly field is increasingly better resolved with satellite-gravity. These recent airborne and satellite gravity datasets provide novel perspectives on Antarctic crustal structure and geodynamic evolution. We review results from some of these surveys over the Gamburtsev Subglacial Mountains, Dronning Maud Land, the Wilkes Subglacial Basin, the Transantarctic Mountains and the West Antarctic Rift System and present gravity modelling outputs of crustal thickness for these regions. We contrast these gravity results with a seismically-derived estimation of Antarctic crustal thickness (Baranov and Morelli, 2013, Tectonophys). Anomalously thick East Antarctic crust lies beneath the Gamburtsev Mountains and parts of Dronning Maud Land (50-58 km). Crustal thickening may stem from the collision of a mosaic of East Antarctic crustal provinces in Meso to Neoproterozoic times (Ferraccioli et al., 2011, Nature), or during younger Edicaran to early Cambrian “Pan-African age” orogenic events. The preservation of such thick crust provides significant support for the high bedrock topography in East Antarctica. Additional flexural uplift along the flanks of the Permian to Cretaceous East Antarctic Rift System helps explain the enigmatic Gamburtsev Mountains. Lithospheric flexure along the flank of the West Antarctic Rift System (WARS) may explain the Transantarctic Mountains (TAM), the longest and highest non-compressional mountain range on Earth. Whether the Wilkes Subglacial Basin also developed in response to lithospheric flexure is debated. Our gravity models image thicker crust beneath the Transantarctic Mountains (TAM) (ca 40 km thick), compared to the relatively thinner crust (30-35 km) beneath the Wilkes Subglacial Basin (Jordan et al., 2013 Tectonophys); this is difficult to reconcile with previous flexural model predictions. Three geodynamic processes could explain the thicker crust beneath the TAM: i) Cambrian-Ordovician subduction and accretion along the East Antarctic craton margin; ii) formation of a Paleozoic to Mesozoic plateau in West Antarctica that collapsed leaving behind a region of thicker crust; iii) extensive Jurassic magmatic underplating related to Gondwana break-up. Gravity modelling helps trace the WARS beneath the West Antarctic Ice Sheet (WAIS). The interior Ross Sea Embayment features 25-28 km-thick crust, while parts of the Amundsen Sea Embayment (ASE) are underlain by 19-23 km-thick crust. Narrow Cenozoic rifts may be interspersed with regions of more distributed Cretaceous extension, explaining the anomalously thin crust and lower Te values beneath the ASE. Major contrasts within the WARS are relevant also for the WAIS as these likely exert a key influence on geothermal heat flux variations, which in turn influence basal melting and ice motion.